Synchronization

Now...what's it good for?

That's awesome. I'm totally tempted to go out and buy five metronomes so I can do it at home. I'll have to put it on the list of things to do on a rainy Saturday after I win the lottery.

Woo. Thats pretty sweet.

Beat phenomena is also really cool. An examples would be to set up two pendulums with a spring connecting them at the base. If you start one pendulum swinging, the amplitude of oscillation will slowly decrease and the other pendulum will slowly start to oscillate. Eventually all of the energy will transfer to the opposite pendulum. They will exchange energy back and forth until all of the energy is dissipated by mechanical methods (friction, etc).

There are really interesting applications of vibrations that people rarely realize unless they are into science. A quick google search of flutter will show you videos of vibration in brings or air plane wings that is dependent on the wind speed. The cool bit is that the vibration is perpendicular to the wind. Really cool stuff.